As polymerization initiators to be used in curing a cationically polymerizable compound, such as an epoxy compound, by applying active energy rays, such as light, electron beams, or X-rays, there have been known triarylsulfonium salts (Patent Document 1), phenacylsulfonium salts having a naphthalene skeleton (Patent Document 2), dialkylbenzylsulfonium salts (Patent Document 3), and sulfonium salts resulting from the introduction of a thioxanthone skeleton into a sulfonium salt (Patent Document 4).